In this editorial, the editor-in-chief thanks those who served the journal in 2018.

This is a list of reviewers who served OE in 2018.

Structured surfaces composed of subwavelength-sized features offer multifunctional properties including antireflective characteristics that are increasingly important for the development of micro-optical components. Here, three-dimensional (3-D) direct laser writing, via two-photon polymerization, is used to fabricate planoconvex spherical microlenses with antireflective structured surfaces. The surfaces are composed of subwavelength-sized conicoid structures, which are arranged fully conformal to the convex surface of the microlenses. The dimensions of the conicoid structures are optimized to effectively reduce Fresnel reflection loss over a wide band in the near-infrared spectral range from 1.4 to 2.2  μm, with a maximum reduction at 1.55  μm. Infrared reflection and transmission measurements are used, in combination with 3-D finite element calculations, to investigate the performance of the microlenses. The experimental results reveal that in the spectral range from 1.4 to 2.2  μm an effective suppression of the Fresnel reflection loss at the convex surface of spherical microlenses can be achieved. The transmittance enhancement is ranging from 1% to 3% for spherical microlenses with antireflective structured surfaces, in comparison to an uncoated reference.

Additive manufacturing fabricates the desired final part by depositing and fusing layer upon layer of the source material and offers both benefits and disadvantages compared to traditional manufacturing. New engineering designs are possible in which a single optimized part with topology can replace several traditional parts. The complex physics of metal deposition leads to variations in quality and to new flaws and residual stresses not seen in traditional manufacturing. Additive manufacturing currently has gaps in knowledge. Mission assurance for the space industry will require: qualification and certification standards; sharing of data in handbooks; predictive models relating processing, microstructure and properties; and development of closed loop process control and nondestructive evaluation to reduce variability. A tailored qualification strategy for additive manufacturing accounts for the manufacturing readiness level, mission risk class, and the knowledge of material properties. Three case studies are presented on the development and qualification of AM for the space industry with the common goals of maturing the technology and improving its reliability.

This paper presents a compact survey of the various material schemes and device structures that have been explored in the quest toward developing light-emitting diodes (LEDs) based on zinc oxide (ZnO) and related II-oxide semiconductors. Both homojunction and heterojunction devices have been surveyed. Material for fabricating these devices has been grown with a number of different techniques, such as pulsed laser deposition, molecular beam epitaxy, metal-organic chemical vapor deposition, and atomic layer epitaxy. This review also features a self-contained introduction to materials science and device processing technologies that are relevant for fabricating ZnO LEDs. These topics include dry and wet etching, contact formation, and optical doping of ZnO. Due to the overwhelming importance of p-type doping of ZnO for making electronic and optoelectronic devices, a separate short section on electrical doping of ZnO is also included. The rest of this paper describes several different attempts at making blue- and ultraviolet-emitting ZnO LEDs. These include simple pn-junction devices as well as more complicated heterostructure devices incorporating charge carrier barriers and quantum wells.

This guest editorial summarizes the Special Section on High Power Laser Ablation.

In the not so distant future, the laser ablation propulsion rockets powered by ground-based high average power laser will challenge the existing techniques of accessing space based on chemical propulsion. Chemical rocket propulsion, which made access to space possible to begin with and continues to be the only technology in use to that end, cannot, however, adequately serve the emerged and quickly growing launch-to-orbit needs of the pico-nano-micro-satellites niche markets. No further technological improvements in chemical rocket propulsion can change its well-known inherent limitations and disadvantages. The ongoing and imminent developments in the satellites’ markets as well as the timeless fundamental need in robust low-cost ground-to-orbit logistics urgently call for adopting a new game-changing technological paradigm for accessing space. By elaborating and expanding on the potentially key role of the laser ablation propulsion launch system in enabling the affordable and safe access to space, this paper logically continues the author’s previous work “Optimizing access to space: ground-to-orbit logistics framework (GOLF)” published in 2017 in the New Space journal.

A ground-based laser system for space debris cleaning requires pulse power well above the critical power for self-focusing in the atmosphere. Self-focusing results in beam quality degradation and is detrimental for the system operation. We demonstrate that, for the relevant laser parameters, when the thickness of the atmosphere is much less than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere produces the phase distortion only. The model thus developed is in very good agreement with numerical modeling. This implies that, by using phase mask or adaptive optics, it may be possible to eliminate almost completely the impact of self-focusing effects in the atmosphere on the laser beam propagation.

Small space debris objects of even a few centimeters can cause severe damage to satellites. Powerful lasers are often proposed for pushing small debris by laser-ablative recoil toward an orbit where atmospheric burn-up yields their remediation. We analyze whether laser-ablative momentum generation is safe and reliable concerning predictability of momentum and accumulation of heat at the target. With hydrodynamic simulations on laser ablation of aluminum as the prevalent debris material, we study laser parameter dependencies of thermomechanical coupling. The results serve as configuration for raytracing-based Monte Carlo simulations on imparted momentum and heat of randomly shaped fragments within a Gaussian laser spot. Orbit modification and heating are analyzed exemplarily under repetitive laser irradiation. Short wavelengths are advantageous, yielding momentum coupling up to ∼40  mNs  /  kJ, and thermal coupling can be minimized to 7% of the pulse energy using short-laser pulses. Random target orientation yields a momentum uncertainty of 86% and the thrust angle exhibits 40% scatter around 45 deg. Moreover, laser pointing errors at least redouble the uncertainty in momentum prediction. Due to heat accumulation of a few Kelvin per pulse, their number is restricted to allow for intermediate cooldown. Momentum scatter requires a sound collision analysis for conceivable trajectory modifications.

We present the formation of micro- and nanostructures on glass surfaces by 15-ns 193-nm ArF laser irradiation during the laser irradiation. The effect of laser fluence and pulse number on the micro- and nanostructure formation is studied. The scanning electron microscopy images of the irradiated surfaces show that the shape and density of the microstructures change with different laser parameters. Our results show that the microstructure formation is a characteristic feature of the ablation threshold of the glass surface at 15-ns ArF laser irradiation. The ordered microstructures are obtained with 500 pulses at 350  mJ  /  cm² laser fluence. Also, the nanostructures can be formed at subthreshold laser fluence. The micro-Raman analysis of the glass surfaces before and after the laser irradiation shows that no phase change has occurred during surface structuring. The obtained compact and ordered micro- and nanostructures or multiscale structured surface can be applied in photonics and surface science.

One of the main figures of merit in laser-ablative propulsion is the specific impulse, I_sp, defined as the impulse per unit weight of fuel, and it is related to the exhaust velocity, v_e, by the acceleration of gravity, I_sp  =  v_e  /  g. Being a key magnitude, I_sp needs to be accurately determined. It is usually inferred from other measurable quantities: the impulse coupling coefficient, C_m, defined as the ratio of the target momentum produced to the incident laser pulse energy, and Q^*, the laser energy consumed per unit weight of ablated target material. Thus, I_sp is calculated as I_sp  =  C_mQ^*  /  g. However, single pulse ablated mass leading to Q^* is in the nanogram scale and cannot be directly measured by weighting the targets. So, mass loss measurements are performed by analyzing the volumes of the craters produced by a large number of laser pulses. These procedures lead to larger than desired uncertainties in the I_sp values. On the other hand, more precise measurements of I_sp can be carried out from the direct measurement of the exhaust velocity of the ejected particles by interferometric methods. In this work, a system based on a Nomarsky interferometer has been set up for the time-resolved diagnostic in the nanometric scale of laser ablation plumes. The performance of the implemented system was first validated by measuring the I_sp produced by aluminum targets and solid propellants based on metal/salt mixtures. The C_m dependence on laser parameters and binary composition of these propellants have been determined in previous works with a torsion pendulum and a piezoelectric sensor. Once the interferometer performance is characterized, the I_sp produced by solid propellants composed of metal (Zn) and metal oxides (ZnO) matrices doped with nanoparticles of different materials is determined and compared.

This paper presents the method of fabricating a frequency-selective surface (FSS) filter for terahertz frequency by short- and ultrashort laser ablation process. The FSS consists of a capacitive screen made up of the copper metallic structure supported by a Teflon dielectric substrate. The dielectric properties of the Teflon substrate at terahertz frequencies were evaluated using THz-time-domain spectroscopy technique. The numerical simulation of the designed structure was performed using Computer Simulation Technology microwave studio software with unit cell configurations and the resonance was observed at 0.135 THz. The desired metallic microstructure was created in copper using an 8-ns solid-state Nd: YAG and 150 fs Ti:sapphire laser operating at the second harmonic wavelength of 532 and 800 nm, respectively. The proposed structure provides polarization-independent operation. The experimental verification of the fabricated FSS using femtosecond ablation process was done using THz-TDS technique, and it is observed that the measured data well match with the simulated result.

Adaptive optics (AO) systems have been used in many applications, such as ground-based astronomical telescopes for improving the resolution by counteracting the effects of atmospheric turbulence. However, the traditional AO system model is not good at dealing with noise interference in the closed-loop correction. Therefore, our paper proposes a subspace system identification method based on errors-in-variables to build an accurate dynamic model of the AO system from measurement data with closed-loop system identification. Experimental simulation results show that the root mean square of the residual wavefront is smaller than that of the traditional method, whether photon noise and camera readout noise exist or not. We conclude that the identified model has good accuracy and noise disturbance compensation ability to deal with dynamic wavefront correction compared with the traditional method.

A gray-level intensity image can be employed as a host image for hiding a watermark image to protect information security. Past research works demonstrate that a gray-level hidden image can be embedded into the host image by a digital phase-only holography method. However, the fidelity of retrieved watermark image from the host image is not very satisfactory and the host image quality is degraded due to the insertion of external data bits, especially when observers focus on the saliency regions in the host image. To address this problem, we propose a steganography method based on digital holography and the saliency map of the host image. First, we calculate the hidden capacity (number of bits to be replaced) for each host image pixel based on the weighted sum of pixel intensity and saliency value. Next, a multilevel phase-only digital hologram of the watermark image will be calculated by the Gerchberg–Saxton method under the constraint of the hidden capacity of host image. Finally, we embedded the multilevel phase-only digital hologram into the host image by replacing a corresponding number of bits in each pixel. In this way, the host image can preserve good image fidelity for its saliency regions even if we hide a large amount of digital hologram data into the host image. The experimental results show that the quality of retrieved watermark image from the host image and the quality of saliency regions in the watermarked host image, in our proposed scheme, are superior to the state-of-the-art works reported.

As a signal processing theory, compressive sensing (CS) breaks through the limitations of the traditional Nyquist sampling theorem and provides the possibility to solve the high sampling rate, large data volume, and real-time processing difficulties of traditional high-resolution radar. Based on the theory of single-pixel cameras, an array detection imaging system is built, and main structural parameters are analyzed. The simulation experiment of a simple target is organized to show that the number of measurements can be reduced by achieving the parallel operation through increasing the number of detectors. When the target changes, it is found that the sparsity problem has a great influence on the number of measurements. Therefore, an improved method is proposed using the structure flexibility of fiber array and detectors, which can reduce the number of measurements simultaneously while decreasing the number of detectors, which is superior to the original method.

Spectral computed tomography (CT) can reconstruct scanned objects at different energy-bins and thus solve the multimaterial decomposition (MMD) problem. Because the linear attenuation coefficients of different basis materials may be extremely close, the decomposition problem is often ill-conditioned. Meanwhile, traditional material decompositions with image-domain algorithms are usually voxelwise based. Therefore, these algorithms rely heavily on image quality. Ring artifacts often exist in the reconstructed images of spectral CT due to the inconsistency feature of energy-resolved detectors and beam-hardening effect. Considering the enlargement of the receptive field and taking advantage of the modeling ability of convolutional neural networks in deep learning, we proposed a convolutional material decomposition algorithm to solve the MMD problem through a basis of patches instead of pixels of the spectral CT images. Simulations and physical experiments were performed to validate the proposed algorithm, and its quality was compared with a traditional MMD algorithm in the image domain. Results show that the proposed method achieves good accuracy, reduces mean squared errors by one to two orders, and exhibits robustness in the MMD of spectral CT images even in the case that obvious ring artifacts is presented.

Long-term robust tracking remains a challenging problem in visual object tracking. Based on the popular tracking-by-detection framework, we propose an approach named real-time long-term correlation tracking by single-shot multibox detection (RLCT-SSD), in which tracking and detection work in parallel and cooperate together. Our algorithm consists of two parts: a tracking module is expected to track in real time and accurately, whereas a detection module is responsible for verifying the tracking results at intervals, and redetecting if necessary. The detector updates the tracking module when the tracking result is unreliable. We introduce a motion model on the basis of fast discriminative scale space tracker to design our tracking module. The detection module is based on the single-shot multibox detector algorithm. To further reduce the computational cost, we use the ShuffleNet as our base network of detection. The experimental results on OTB2013 and OTB2015 demonstrate that our RLCT-SSD performs favorably against most state-of-the-art trackers and achieves long-term accurate tracking running in real time.

In deep ocean applications, a camera’s viewing range is related to its low-light-level performance. Hence, a high-resolution ultra-low-light-level digital still camera (thereafter L3DSC), designed from the ground up for deep ocean autonomous underwater vehicle (AUV) imaging, is presented. A high-resolution (1024  ×  1024) electron-multiplying CCD (EMCCD) is adopted as an image sensor to ensure low-light capability. A thermo electric cooler (TEC) is employed to lower image sensor temperature to promote low-light-level performance. Totem pole circuits that are able to generate 50-V pulse wave at 10 MHz are implemented to drive the EM pin, and power consumption of the circuit is optimized. An EMCCD digital image is buffered in a field-programmable gate array (FPGA) and then transferred through USB interface to a solid-state drive (SSD), which is installed in the image storage unit. The camera is controlled by AUV via Ethernet, and image data stored in the camera could be downloaded via the same interface after AUV retrieved from the sea. The camera module is mounted into a 6000-m depth-rate titanium alloy housing. The diagonal field of view is measured to be 58.4 deg in air. Experiments show that the minimum scene illumination of L3DSC is better than 5  ×  10^−  4  Lux; underwater imaging distance is longer than 10 m, and total image data capacity is 200 gigabytes. These results demonstrate the camera’s low-light-level imaging performance and feasibility for AUV applications.

Microgroove cutting by fly-cutting is studied to obtain the optimized parameters to get a smooth groove with less burr and chip. We analyzed the reason for the second groove and reduced the cutting times to fade the effect of machine reposition error. Introducing additional empty cut further reduced the burr and chip. To get the ideal micropyramid arrays, all factors must be considered and controlled. Finally, processed pyramid arrays with the depth of 1  μm whose structure distributed the entire surface evenly and with no obvious chip.

Compressive sensing ghost imaging (CSGI) is an imaging mechanism that can nonlocally obtain an unknown object’s information with a single-pixel detector by the correlation of intensity fluctuations. In the practical research and application of CSGI, object detection plays a crucial role in real-time monitoring and dynamic optimization of speckle pattern. We demonstrate, for the first time to our knowledge, how to solve the low-resolution and undersampling problems in CSGI object detection. The method we use is to combine generative adversarial networks (GANs) with object detection systems. The robustness of the object detection model can increase by generating reconstructed images of different resolutions and sampling rates for training. The experiment results have verified that the mean average precision of CSGI object detection using GANs has been improved 16.48% and 2.98% on MSCOCO 2017 compared with two traditional learning methods, respectively.

We present an optical concept for imaging sensor systems, designed to considerably reduce the sensor’s image information loss in cases of laser dazzle, based on the principle of complementary bands. For this purpose, the sensor system’s spectral range is split in several (at least two) spectral channels, where each channel possesses its own imaging sensor. This long-known principle is applied, for example, in high-quality three-sensor color cameras. However, in such camera systems, the spectral separation between the different spectral bands is too poor to prevent complete sensor saturation when illuminated with intense laser radiation. We increased the channel separation by orders of magnitude by implementing advanced optical elements. Thus, monochromatic radiation of a dazzle laser mainly influences the dedicated transmitting spectral channel. The other (out-of-band) spectral channels are not or—depending on the laser power—only hardly affected. We present our system design as well as a performance evaluation of the sensor concerning laser dazzle.

Nonuniform refractive index distributions in transparent mediums are of interest as it gives rise to a modification of the probe light beam passing through such mediums. Various properties of the probe beam can be used to quantify the modification happening to the probe beam. One of these properties is the deflection of the beam. This could be used to map and quantify the spatiotemporal evolution of refractive index distribution in such mediums. The deflections could be measured by imaging the deflection of structured line pattern projected through such a system. We describe the development of a compact, portable device for mapping of refractive index distributions as well measurement of the diffusion coefficient of liquid solutions. The method and device are demonstrated by the real-time display of the refractive changes as well as measurement of diffusion coefficients in diffusing binary liquid solutions.

Adaptive optics (AO) is receiving wide applications for diffraction-limited imaging in astronomical observations and biomedical research. In an AO system, the deformable mirror (DM) is commanded to correct wavefront errors (WFEs) and is a key component. However, a DM may not be perfect, and it may have strong static WFEs, which may be inherited from the DM manufacturing process or be induced by external factors such as the vibration during the transportation of delivery, which must be corrected before it is integrated into an AO system. To correct this static WFE, we introduce a simple approach. Our approach is based on a two-beam Michelson interferometer configuration, in which a high-quality flat mirror is used as a reference mirror whose surface wavefront can be copied to that of the DM, and thus the DM WFEs can be effectively corrected through an optimization procedure. Our experiments demonstrated that this approach is simple to conduct. Using an iteration optimization approach, the Strehl ratio of the DM can be improved from the initial 0.198 to the 0.997, within 30 min of the optimization run.

The lidar velocimetry is an airspeed sensor for the application of the lidar remote sensing technology to airborne air data systems. This study discusses a 1.55-μm all-fiber continuous-wave coherent laser radar (CLR) system, with a balanced photodetector as receiving frequency discriminators, developed by the Ocean Remote Sensing Laboratory, Ocean University of China. The system exhibits advantages including a compact structure, light weight, and high precision. The coupling efficiency of light from free space to single-mode fiber is analyzed, and the optimal value of the relative aperture of the coupling lens is given (d  /  f  =  0.2107). The expression of carrier-to-noise ratio is simulated based on the parameters of the components used in the system to obtain the optimum local oscillator power (0.724 mW). The presented result of the hard target experiment and vehicle-mounted experiment in August 2018 indicates that the correlation coefficient between the lidar measured value and actual speed value is better than 0.99, relative error is less than 2.6%, and standard deviation is less than 0.22  m  /  s. The aforementioned validation results indicate that the CLR system exhibits stable and reliable performance and the potential to measure aircraft airspeed.

Experimental realization of a fiber Bragg grating-based compact, high-sensitive, and fully packaged single-axis accelerometer has been demonstrated. To verify experimentally observed results, simulation has been carried out for the performance comparison of packaged single-axis accelerometer with the cantilever of various materials. The experimental demonstrations have been conducted at unknown seismic vibrations of random frequencies as well as the vibrations of known frequencies. The experimental results for the reported seismic events (i.e., footprint and hammering) demonstrate that the developed fully packaged accelerometer possesses good response to the signals with random vibrations. The developed accelerometer holds a high sensitivity up to 88 pm/g in the broad bandwidth. Such a compact fully packaged accelerometer with improved sensitivity can be deployed at any civil engineering structures, such as highways, bridges, dams, tunnels, pipelines, and aeronautical platforms for broadband dynamic monitoring of the random vibrations ranging from 5 to 100 Hz.

We propose a simple scheme to perform real-time and rapid measurements of the loop gain and bandwidth of a phase-locked loop (PLL) for mode-locked lasers. By implementing the proposed scheme, the loop gain and bandwidth of a laser-based PLL can be directly and accurately characterized in real time without breaking the original loop. The experimental loop-gain and bandwidth measurement results were found to precisely agree with the simulation results, thereby confirming that the proposed scheme is accurate and reliable. This scheme, with its simple configuration, can be easily expanded to other laser-locking systems that require precise measurements of the loop gain and bandwidth.

In our study, a deviation extraction method is introduced to obtain subtle deviation signals from structural idealized edge profiles. The deviations are employed to reconstruct an analysis matrix that consists of global translations along selected edge profiles, and then a singular value decomposition-based approach is proposed to extract valuable variations from the calculated analysis matrix. To avoid noises from textured edge profiles, a colorization optimization approach is applied to remove variations because of the textures and turn real image stripes into ones that satisfy the constant edge profile assumption more closely in the deviation extraction process. Two practical experiments are conducted to demonstrate the effectiveness and potential applications of our proposed method. The dynamic properties of a lightweight beam and a sound barrier are analyzed successfully by using high-speed videos.

A dual-biprism-based stereo camera system, with an imaging model derived from geometrical optics analysis, is proposed for detection of defects on the external and internal surfaces of pipelines. The proposed system, which comprises two biprisms, a lens, a single camera, and common light sources, overcomes the difference in the camera specifications and synchronization mechanism of conventional two-camera systems. Moreover, different fields of views can be obtained by adjusting the angles of the two prisms and the distance between the dual-biprism and the camera. Measurements can be made even for regions in narrow and small areas using a coaxial illumination and speckle-based three-dimensional digital image correlation method. After a typical experiment performed to verify the efficacy of the proposed system, three different types of defects were accurately detected: (1) a local indentation on the external pipe wall; (2) a weld defect in the pipeline that was measured, and the shape and location retrieved after image processing; and (3) a crack on the internal surface of the pipe determined based on strain calculation. These results validate the proposed system as an efficient and convenient tool for defect detection.

In order to improve the stability of the calibration method based on a single parallel plane, we propose a robust calibration method based on the calculation of traditional homography matrix and Gaussian imaging model. This method first uses a convenient and accurate method to calculate the physical focal length of the entire microscope optical system, and then uses the Gaussian imaging model to calculate the effective focal length and intrinsic matrix of the camera. Then, the camera’s extrinsic parameters can be easily calculated from the homography matrix. Finally, all camera parameters can be optimized along with the lens distortion through a nonlinear optimization process. In this method, only two images of the parallel calibration plate need to be captured to complete the calibration. The proposed method overcomes the problem of instability of the existing microscope calibration methods and significantly improves the robustness and accuracy of the whole calibration process.

We provide an overview of the method of confocal mirror design and report advances with respect to pupil imagery. Two real ray-based conditions, Y⁺  =    −  Y⁻ and ΔY  =  0, for the absence of linear astigmatism and field tilt are presented. One example illustrates the design of a system confocal of the object and image, and another illustrates the design of a system confocal of the pupils. Stop shifting formulas are provided. Three three-mirror anastigmatic systems further illustrate the method.

To improve the effective deformation of the unimorph deformable mirror (DM), ion beam figuring (IBF) is applied for manufacturing the initial surface figure. However, heat produced during IBF can cause comparable internal stress and thermal deformation on the adhesive layer, which causes the mirror’s figure to be unsatisfactory. Consequently, the whole manufacturing process, especially thermal distribution on the adhesive, should be monitored and controlled. A 19-unit thin-layer unimorph DM is adopted as the object to simulate and measure the distribution law of temperature field on the adhesive during the IBF process. IBF parameters are optimized according to the characteristics of the adhesive and the manufacturing simulation results. With a ϕ3-mm diaphragm integrated on IBF, the processing temperature is reduced below the adhesive’s glass transition temperature T_g. Actual figuring results report over 50% reduction on both peak-to-valley (PV) and root-mean-square (RMS) of DM with the value from 310 to 141 nm and from 38 to 17 nm, respectively. This precision has satisfied the self-corrected requirement of general unimorph DMs, which is usually <20  nm RMS. The voltage fitting coefficient of self-correcting for most electrodes is reduced to <0.05, which means the significant improvement in the effective deformation and correction ability.

Coating techniques are investigated for accurate shape correction of Kirkpatrick–Baez (KB) mirrors. Au and Pt have been used as coating materials to obtain elliptical KB mirrors from flat or cylindrical Si substrates. However, due to large differences in the thermal expansion coefficients between Au (or Pt) and Si, thermal stress can be induced not only when coating the substrate, but also during use when focusing synchrotron beams. Si is a promising alternative for profile coating because not only the same material as substrate can induce much smaller thermal stress, but also Si is generally smooth. The microstructure and roughness of 1000- and 2000-nm-thick Si coatings deposited at two different pressures (0.133 and 0.266 Pa) are explored. When the thickness increases from 50 to 2000 nm, the film microstructure and surface morphology change and root mean square roughness increases from 0.164 to 0.232 nm. The increase in Si coating thickness contributes the most to power spectral density curves when the frequency is between 1 and 20  μm^−  1. No obvious changes resulting from intrinsic stress are observed among the samples when the Si coating thickness is increased from 50 to 2000 nm.

The reversible logic gate is a great choice for designing various types of optical processors due to so many advantageous aspects of it; such as low power consumption and the ability of data recovery. At first, the optical circuit of Fredkin gate, a universal reversible logic gate, is designed theoretically using a semiconductor optical amplifier (SOA)-based polarization switch, and the performance of the circuit is studied using simulated results. The frequency-encoded binary data are used to execute the operations. Subsequently, we propose the techniques of developing NOT, AND, OR, EX-OR, EX-NOR gates, and full adder circuit using reversible Fredkin gates. Finally, we develop an optical arithmetic logic unit using the proper combination of reversible Fredkin gates. The theory of nonlinear polarization rotation of the probe beam in the presence of the pump beam in SOA is adopted here. The simulated input–output spectra of the optical circuits enhance the admissibility of the proposed concept.

In the past few years, a new type of camera has been emerging on the market: a digital camera capable of capturing both the intensity of the light emanating from a scene and the direction of the light rays. This camera technology called a light-field camera uses an array of lenses placed in front of a single image sensor, or simply, an array of cameras attached together. An optical device is proposed: a four minilens ring that is inserted between the lens and the image sensor of a digital camera. This device prototype is able to convert a regular digital camera into a light-field camera as it makes it possible to record four subaperture images of the scene. It is a compact and cost-effective solution to perform both postcapture refocusing and depth estimation. The minilens ring makes also the plenoptic camera versatile; it is possible to adjust the parameters of the ring so as to reduce or increase the size of the projected image. Together with the proof of concept of this device, we propose a method to estimate the positions of each optical component depending on the observed scene (object size and distance) and the optics parameters. Real-world results are presented to validate our device prototype.

An achromatization method optimized for dual-channel imaging is developed. Dichroic mirrors are employed to split and recombine narrowband signals, and separation between catoptric components is used to minimize the longitudinal chromatic shift. An achromatic system based on this principle could be built from singlet lenses, since refractive element properties such as dispersion and power are not utilized to optimize wavelength-dependent performance. To demonstrate the validity of the proposed solution, a prototype miniature fluorescence microscope optimized for two emission lines of acridine orange (525 and 650 nm) is built. To reduce the cost and accelerate assembly, the system is built from commercially available optical components. The optical train consisted of two plastic singlet lenses combined with a pair of dichroic mirrors. Optical performance of the prototype is evaluated by imaging a bar line target at both design wavelengths. To demonstrate the potential of the proposed design strategy, the achromatic system prototype is used to measure a two-part white blood cells differential count on a venous blood sample. Data from the prototype fluorescence microscope are compared against results from a commercially available blood analyzer, and the difference between both instruments is within 20%.

When infrared gas detection technology is applied to monitor gases, the detection accuracy decreases due to the influence of systematic noise. The noises are filtered using traditional denoising methods; however, these methods affect the useful signal. An infrared photoelectric signal is analyzed using the parameter-tuning stochastic resonance method. Based on the parameter normalization frequency and the output SNR tuning bistable system parameters, the system reaches the optimal resonance state, and the signal after denoising is obtained using the Runge–Kutta algorithm. The numerical simulation is conducted on the periodic photoelectric signal with input SNR of 10 dB. It is determined that the method is suitable for the photoelectric signal detection with a frequency from 1 to 10 Hz and that the output SNR is increased to 20 dB in the optimal resonance state. The comparison experiment results show that this method can improve the original signal more effectively than other denoising methods, improving the SNR to 25 dB. In the CO₂ standard gas concentration range of 0% to 5%, the maximum relative error of inversion is ±7  %  , and the correlation between the inversion result and the standard concentration is 0.9982. This suggests that the proposed method is effective for infrared photoelectric signal detection under strong noise conditions.

The high reflectivity design of the one-dimensional (1-D) heterostructure photonic crystal is investigated in infrared regions 3 to 5 and 8 to 12  μm. First, the reflection characteristics of the light waves in the 1-D periodic photonic crystal are systematically analyzed. Calculations using the transfer matrix method and simulations verify that the λ  /  4 dielectric film is the optimum option for achieving the ideal optical reflectivity in a certain wavelength range. The merging of the even-layered high-reflective photonic crystals can be used to expand the gap of the infrared band. In contrast, a combination of the odd-numbered composite photonic crystals can be used to achieve a high transmittance at a wavelength of 10.6  μm with a larger gap in the infrared region. Accordingly, an enhanced photonic crystal is designed using silver metal at the interface and even-numbered dielectric films of Ge and ZnS with 23 layers of the 1-D metal. The simulated reflectivity of this photonic crystal ranges from 95% to 99.9% in wavelength ranges 3 to 5 and 8 to 14  μm. This design significantly reduces the number of layers required for forming a 1-D high-reflective photonic crystal in the infrared bands 3 to 5 and 8 to 12  μm.

An algorithm for selecting the correct starting point for computer optimization of a two-mirror scanner with a meniscus lens is developed for use in laser machining. This algorithm performs joint analysis of aplanatism condition fulfillment and the field characteristics of the ghosts reflected from the meniscus surfaces back into the scanner mirror space. For integrity, all equations and estimates are given with respect to a major parameter: the curvature of the input surface. The second powerful tool for the optimization is the distance from the meniscus to the scanning mirrors, though its applicability is significantly limited by design considerations. Several scanner variants used to perform basic laser machining processes at various power levels are considered in detail. It is found that, for a fixed output numerical aperture, compacting the scanner always improves its optical performance. In general, compacting is an alternative to using scanners in systems with high-power laser sources. The results of this work are valid for any optical material and wavelength and are particularly relevant for systems based on CO₂ lasers, in which such scanners remain widely used.

A constellation-designed 32QAM method is proposed using geometric shaping (GS) and probabilistic shaping (PS). In addition, a 50-Gbaud polarization division multiplexed multispan optical fiber link for the proposed scheme is set up. The simulation results show that when the net data rate reaches 211 Gbit/s, the proposed 32QAM with GS and PS is capable of achieving about 0.1- and 0.3-dB OSNR gain over PS-32QAM and regular 32QAM (R-32QAM), respectively. When 213 Gbit/s net data rate transmission is achieved, the use of the proposed 32QAM can increase the transmission length by ∼60  km over PS-32QAM and ∼210  km over R-32QAM. Moreover, when OSNR is set to 19 dB and signal bandwidth is 40 Gbaud, the net data rate of the proposed 32QAM is 2 and 7.5 Gbit/s larger than PS-32QAM and R-32QAM, respectively.

Recently, visible light communication (VLC) has been widely used in indoor positioning. Considering that conventional VLC-based positioning (VLP) algorithms are susceptible to interference and cannot adapt well to the complex indoor location environment, we propose a high-precision indoor three-dimensional positioning algorithm based on VLC and fingerprinting using fusion of K-means and random forest algorithms. Unlike the trilateration techniques based on received signal strength, the mentioned system does not depend on the model parameters and is robust to external interference. Besides, the proposed system introduces the concept of VLP kernel to compress the fingerprint database and shorten the training time. And we have validated the feasibility of our proposed VLC-based positioning system using fingerprinting in the real indoor VLP environment. Both simulation and experiment results verify that our proposed algorithm delivers satisfactory performance in terms of real-time ability and positioning accuracy, both of which are crucial for the performance of an indoor positioning system. Therefore, our proposed positioning system with strong interference immunity is a promising candidate for future indoor positioning applications.

A type of homodyne coherent optical communication system is proposed on the basis of a reflex electro-optic modulator. To measure the performance of this system, the effect of a 100-Gb/s dual polarization-quadrature phase-shift keying system is simulated using OptiSystem software and MATLAB code. The compensation for chromatic dispersion, polarization mode dispersion, and nonlinear impairment is carried out by means of digital signal processing. The results show that the performance of the reflective homodyne coherent optical communication system is feasible. It solves the phase-shift problem caused by the frequency offset between the transmitter and local oscillator as well as the laser linewidth. The technique may be useful in practical engineering systems.

We propose a 2-D visible light positioning system based on the artificial neural network (ANN), where the light-emitting diodes are grouped into blocks and the block coordinates are encoded with under-sampled modulation. A camera is used to decode the block coordinate in the receiver. The receiver’s position is approximately and precisely estimated using the decoded block coordinate and a typical back propagation ANN, respectively. The experimental results show that the proposed scheme offers a mean positioning error of 1.49 cm.

Disorder and scattering in photonic systems have long been considered a nuisance that should be circumvented. Recently, disorder has been harnessed for a rapidly growing number of applications, including imaging, sensing, and spectroscopy. The chaotic dynamics and extreme sensitivity to external perturbations make random media particularly well-suited for optical cryptography. However, using random media for distribution of secret keys between remote users still remains challenging since it requires the users have access to the same scattering sample. Here, we utilize random mode mixing in long multimode fibers to generate and distribute keys simultaneously. Fast fluctuations in fiber mode mixing provide the source of randomness for key generation, and optical reciprocity guarantees that the keys at the two ends of the fiber are identical. We experimentally demonstrate the scheme using classical light and off-the-shelf components, opening the door for a practically secure key establishment at the physical layer of fiber-optic networks.

Quantum key distribution (QKD) is a method for establishing secure cryptographic keys between two parties who share an optical, “quantum” channel and an authenticated classical channel. To share such keys across the globe, space-based links are required and in the near term these will take the form of trusted node, key management satellites. We consider such channels between two nanosatellite spacecraft for polarization entanglement-based QKD, and the optical channel is described in detail. Quantum channels between satellites are useful for balancing keys within constellations of trusted node QKD satellites and, in the future, may have applications in long-distance qubit exchange between quantum computers and in fundamental physics experiments. The nanosatellite mission proposed uses an optical link with 80-mm diameter optical terminals. If such a link could be maintained with 10-μrad pointing accuracy, then this would allow QKD to be performed for satellite separations up to around 400 km. A potential pointing and tracking system is also described although currently this design would likely limit the satellite separation to 100 to 150 km.

We propose a quasidistributed optical fiber-sensing system based on optical time-domain reflectometry (OTDR). The quasidistributed optical fiber-sensing system can be used to monitor the joint opening displacement or crack opening displacement of the face slab of concrete-faced rockfill dams (CFRDs). The crack sensors, which are based on the sensing principle of linear macrobending loss of optical fiber, are connected in series in each optical fiber link. In addition, multiple links can be connected with a single OTDR to construct an integrated monitoring system. Hence, the quasidistributed system overcomes some drawbacks of the distributed monitoring system, such as limited measurement range, fragility of fibers attached to the structural surface, and the nonlinear relationship between displacement and macrobending loss of optical fiber. To develop an optimal design plan for the monitoring system, some experiments are conducted to verify the independence of crack sensors and the minimum length of the connecting fiber between sensors. The analysis of the experiment results positively supports the application of optical loss in real structural monitoring.

Visible light communication, as a promising future wireless communication technology, has been considered as an attractive solution for indoor positioning systems. An indoor visible light positioning system based on cooperative localization is proposed to support illumination and positioning simultaneously. By cooperation among agents, the positioning system can work in a harsh environment, where a two-light-emitting diodes positioning scheme is proposed as an example specifically. The Bayesian framework and factor graph are introduced to describe the location problems, while the sum-product algorithm is adopted to deal with the positioning process and determine the receivers’ location. Moreover, simulations are carried out to demonstrate that the proposed system can achieve centimeter-scale positioning accuracy, and the feasibility is also verified.

Poincare sphere (PS) representation for all possible modes of a few-mode optical fiber is presented. The fiber that can support up to two scalar modes is considered for the study. The polarization aspects of first-order linearly polarized degenerate modes and their possible combinations that generate inhomogeneously polarized vector vortex modes (VVMs) are discussed using Stokes analysis. A class of VVMs such as spiral and hybridly polarized modes, apart from well-known radial and azimuthal vector modes, are generated by the linear combination of first-order orthogonal linearly polarized modes with tilted polarization vector. All homogeneously polarized fiber modes are accommodated on standard PS, and inhomogeneously polarized vortex modes are accommodated on a pair of higher order PS. The location of VVM on higher order PS is justified by the positions of orthogonal linearly polarized modes on standard PS.

The size of the field of uncertainty (FOU) is an important performance indicator of spatial optical communication systems and is closely related to the line-of-sight (LOS) pointing accuracy. Considering the aircraft-ground laser communication as the research background, the initial pointing angle of the LOS of communication is obtained using the coordinate transformation matrix, and the factors that affect the LOS pointing accuracy error are analyzed. The Kalman filtering technique is introduced into the LOS pointing system to complete system modeling and simulation analysis. The simulation results show that the position, attitude angle, and other data fluctuations of global positioning system/inertial navigation system composite systems can be smoothed effectively by adding a Kalman filter to the LOS pointing system, and the prediction function of the filter can reduce the influence of dynamic lag. The proposed approach improves the accuracy of the LOS pointing, reduces the size of the FOU, and provides reference and guidance for the design of space optical communication systems.

An all-fiber midinfrared supercontinuum with 20-dB spectral coverage from 1.9 to 4.6  μm is demonstrated with a record power ratio beyond 3.8  μm. The supercontinuum is generated in a piece of single-mode ZBLAN (ZrF₄-BaF₂-LaF₃-AlF₃-NaF) fiber pumped by a broadband, single-mode thulium-doped fiber amplifier. Under the optimized pulse repetition rate and ZBLAN fiber length, the output spectrum has a good flatness. The power ratio beyond 3.8  μm is measured to be over 30% when the average output power reaches 1.11 W. Based on the all-fiber configuration, we have provided a compact, reliable, and promising supercontinuum source for further spectral extension into the midinfrared region where a high long-wavelength ratio is preferred.

Optical frequency-domain reflectometry and frequency-modulated continuous wave (FMCW)-based sensing technologies, such as LiDAR and distributed fiber sensors, fundamentally rely on the performance of frequency-swept laser sources. Specifically, frequency-sweep linearity, which determines the level of measurement distortion, is of paramount importance. Sweep-velocity-locked semiconductor lasers (SVLLs) controlled via phase-locked loops (PLLs) have been studied for many FMCW applications owing to its simplicity, low cost, and low power consumption. We demonstrate an alternative, self-adaptive laser control system that generates an optimized predistortion curve through PLL iterations. The described self-adaptive algorithm was successfully implemented in a digital circuit. The results show that the phase error of the SVLL improved by around 1 order of magnitude relative to the one without using this method, demonstrating that this self-adaptive algorithm is a viable method of linearizing the output of frequency-swept laser sources.

Greenberger–Horne–Zeilinger (GHZ) state is an essential resource in quantum processing. We propose multiparty measurement-device-independent quantum key distribution (MDI-QKD) protocol based on a fuzzy GHZ state analyzer, which is composed of linear optical components. Our scheme requires only N detectors and N  −  1 polarizing beam splitters to realize an N-party MDI-QKD network. From numerical simulations, the security communication distance is improved to 300 km, and key rate is determined to be higher than 10^−  14. Compared with the previous work, our scheme needs less optical components and detectors, is easier to extend more users, and narrows the gap between theory and practice in multiparty MDI-QKD network.

The availability and reliability of the high data rate space-to-ground optical coherent communication links are critically challenged because the wave front of the signal beam is distorted and impaired when propagating through atmospheric turbulence. Based on the free-space interference of two successive data bits delayed by an unequal-arm-length Mach–Zehnder interferometer (MZI), a pupil-matching time-delay self-homodyne interference differential phase-shift keying (DPSK) optical receiver with 2  ×  4 90-deg optical hybrid is designed for the high data rate space-to-ground optical communication links due to its immunity from wave front impairment. The delayed optical path difference (OPD) in the unequal-arm-length MZI corresponds to the duration of one bit and can be stabilized to below 1000th of the wavelength by the closed-loop feedback control. The maximum system insertion loss is <1  dB. The measurement accuracy of OPD in unequal-arm-length MZI is 0.01 mm by the deramping method from a chirped laser. The double-bit-rate 2.5-  /  5-Gbps DPSK optical receiver has been presented. Parallel and separate atmospheric measurement along the optical communication link is also performed simultaneously. The 2.5-and 5-Gbps optical communication links have already been verified with a ϕ300-mm receiving telescope between two buildings in downtown over a distance of 2.4 km in the worst-case atmospheric conditions. The measured bit-error-rate is better than 10^−  6. Without wave front compensation of the adaptive optics, local oscillator, and optical phase-locked loops, the pupil-matching time-delay self-homodyne interference DPSK optical receiver has great significance for future developments of space-to-ground optical communication links.

We investigate in detail the resonant properties of a two-dimensional dielectric cavity with an equilateral triangle shape, using a numerical integral equations approach and a semiclassical approach. The homogeneous Müller boundary equations are used to calculate the resonant modes of a dielectric triangle in a wide range of frequencies. It is shown that the modes of dielectric triangles localized on families of periodic orbits can be well described in terms of a semiclassical superscar model. Special attention is given to the families of resonances based on the inscribed triangle periodic orbit.

The high efficiency single-longitudinal-mode (SLM) Tm, Ho:YAP laser with ring cavity is proposed. The Tm, Ho:YAP ring laser was run on unidirectional operation using a Faraday rotator and a half-wave plate. A 0.2-mm F-P etalon was employed for wavelength tuning. SLM power could reach 231 mW with the slope efficiency of 23%, M² factor of 1.12, and wavelength tuning range of 2053.62 to 2058.0 nm. This is the first report on the SLM Tm, Ho:YAP laser based on Faraday effect with high efficiency and tunability.

When the beam waist size is on the order of or smaller than a wavelength, the traditional Gaussian solution exhibits noticeable errors in describing a tightly focused laser beam of the fundamental mode. The spatial distributions of the paraxial approximate errors of the Gaussian solution to the paraxial Helmholtz equation, as compared with Sapozhnikov’s exact solution to the scalar Helmholtz equation, are illustrated, tabulated, and discussed systematically. The maximum and average errors of the Gaussian solution are presented for a list of different specified waist sizes of laser beams. We found that the maximum error distribution is of an annular structure and contains the largest error ring on the waist plane. The laser beam waist should be >1.2 wavelengths for a 1% error, while the beam waist should be >3.5 wavelengths for a 0.1% error. This study provides a useful criterion to ascertain whether the traditional Gaussian solution or the exact solution is to be used to describe the fundamental laser beam in optical engineering. The results exhibit potential applications in micronano technology, near-field optical technology, and other fields where tightly focused laser beams are utilized.

The design and performance evaluation of a W-band wavelength division multiplexed-over-optical code division multiple access radio-over-fiber system are presented. The system’s performance introduces expansion in the number of channels and bit rate per channel for millimeter wave signals by optically encoding signals in a multiwavelength transmission system. The performance of the system is verified based on software simulation of three channels that carry six users’ encoded data. The achieved results are measured in bit-error-rate and eye-diagram figures. The obtained results are analyzed for each wavelength channels and each code families used. Bit error rates of 10^−  2 to 10^−  6 are obtained for channels operating at 10  Gb  /  s.

We investigate the performance of half-cosine pulse-shaped hybrid pulse position modulation-orthogonal frequency division multiplexing (HC-PPM-OFDM) under weak (i.e., log-normal) and strong (i.e., gamma–gamma) turbulence free-space optical channel. Joint impact of frequency and time offset on the performance parameters such as intercarrier interference, a symbol to interference ratio, and bit-error rate are also investigated. This research aims to improve the spectral efficiency performance of traditional PPM through the proposed hybrid technique. Further, the proposed HC-PPM-OFDM technique is compared with pulse position modulation-minimum shift keying (PPM-MSK), PPM, and hybrid PPM-OFDM. Obtained results show comprehensive improvement in the performance parameters under both weak and strong atmospheric turbulences. In addition to the above results, the upper-bound channel capacity expression is derived for log-normal as well as gamma–gamma turbulence channel models. Obtained results reveal that the proposed modulation technique outperforms PPM and PPM-MSK techniques under all turbulence conditions at a marginal cost of power penalty.

The first- and second-order fractional-order proportional integral (FOPI) controllers based on the Al-Alaoui operator and the continued fraction expansion method are designed and applied to the closed-loop control system of a resonant optical gyro. The phase margin method is used to tune the control parameters. The characteristics of the unit step responses of the integer order proportional integral (IOPI) and the FOPI controllers are calculated and compared. Responses to perturbations at different positions of the closed-loop control system are simulated. Results show that for the resonant optical gyro, the FOPI closed-loop control system has much better unit-step-response performance and noise-resistance ability than the IOPI control system, which is important for improving the dynamic response characteristics and the robustness of the gyro’s feedback system and enhancing the detection ability of the gyro. Experiments on a resonant optical gyro experimental system also verify the advantages of the FOPI controller in step response and bias stability.

We report on experimental observations of a phenomenon whereby optical coupling of a high-power laser to a photonic subsystem locks a silicon microring’s resonance if temperature of the photonic device is allowed to drift, causing the thermal tuning control to fail. This is the first report of such a failure mechanism, to our knowledge. We refer to this effect as “optical hijacking” of the resonator wavelength. We demonstrate this effect showing that a ring resonator whose resonance drifts due to an increase in chip temperature can be locked to a normally nonresonant wavelength by injecting high optical power to the system leading to this “optical hijacking” effect. Additionally, an analytical description of this effect is presented.

Flexible perovskite solar cells (pero-SCs) have drawn much attention due to their high efficiency and low cost. Indium tin oxide (ITO) and fluorine-doped SnO₂ as the most commonly used transparent electrodes in pero-SCs, unfortunately, exhibit poor mechanical stability in flexible devices. We demonstrate flexible pero-SCs based on an ultrathin Au electrode with perfect performance of transmittance, conductivity, and mechanical stability. The ultrathin Au electrode with the thickness of 7 nm is prepared on the SU-8/Ca modified substrate, and the Volmer–Weber growth mode of deposited metal film has been suppressed successfully by the chemical and physical bonding from the SU-8/Ca hybrid modification layer. The ITO-free flexible pero-SCs based on ultrathin Au electrodes achieve the power conversion efficiency of 11.44%, and nearly 82% of the initial efficiency remains after approximately 2400 bending cycles, demonstrating the high flexibility and mechanical stability of the ultrathin Au electrode as well as the ITO-free pero-SCs.

A two-junction micro p+np+ silicon avalanche-mode light-emitting device (Si AMLED) is analyzed for its dispersion characteristics, which generally resulted in different wavelengths of light (colors) being emitted at different angles from the surface of the device. The SiAMLED is integrated into on-chip bipolar radio frequency-integrated circuitry at micron dimensions. LEDs have high-frequency modulation frequencies reaching into the GHz range. Such devices, which are of micron dimension, operate at 8 to 10 V, 1  μA to 2 mA. The emission wavelength is in the 450- to 850-nm range, emission spot sizes are about 1  μm², and emission intensities are up to 200  nW  .  μm^−  2. The observed geometrical-chromatic dispersion characteristics range from 0.01 deg  /  nm wavelength for green radiation at a 5 deg exit angle to the normal of the device to 0.16 deg  /  nm wavelength for blue radiation at a 60 deg exit angle to the normal of the surface of the device. The high dispersion characteristics of the emitted radiation are attributed to the positioning of the optical source ∼1  μm subsurface to the silicon–silicon oxide interface, as well as to the high-refractive index differences between silicon and the surrounding lower refractive index silicon oxide layers. It is believed that the identified dispersion characteristics will have interesting and futuristic on-chip electro-optic applications for on-chip micro-optical wavelength dispersers, futuristic optical communication demultiplexers, along with on-chip microgas and biosensor applications.

Generation of tunable greenish–yellowish–reddish luminescence mediated by energy transfer in Tb^3  +  /  Sm^3  +-codoped fluorogermanate glass under UV light excitation is reported. Fluorescence emissions around 487, 542, 561, 582, 600, 620, 645, 650, and 706 nm were obtained in double-doped samples under 355- and 375-nm excitation wavelengths. The relative intensity of the Sm^3  + and Tb^3  + ions’ individual distinctive emissions depends upon the excitation wavelength, relative ions concentration, and the energy transfer of Tb^3  + to Sm^3  + mechanism, which lead to the tonality tunable overall luminescent color. The CIE-1931 chromaticity coordinates of the overall luminescence resided in the range (0.31; 0.59) to (0.62; 0.37). Time decay measurements indicated efficient Tb^3  + to Sm^3  + energy-transfer process with a substantial decrease (1.65 to 0.85 ms) of Tb^3  + emitting levels due to the presence of Sm^3  + ions in the host matrix.